1. Field of the Invention
The present invention relates to a positioning system, such as may be used to position a moveable object table in three degrees of freedom. More particularly, the invention relates to the use of the positioning system in a lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second, movable object table for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate.
2. Description of the Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The illumination system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam of radiation. In addition, the first and second object tables may be referred to as the “mask table” and the “substrate table”, respectively.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (comprising one or more dies) on a substrate (silicon wafer) which has been coated with a layer of radiation-sensitive material (resist). In general, a single substrate will contain a whole network of target portions which are successively irradiated via the mask, one at a time. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—which is commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally<1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
In general, apparatus of this type contained a single first object (mask) table and a single second object (substrate) table. However, machines are becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO 98/28665 and WO 98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial metrology steps on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughout, which in turn improves the cost of ownership of the machine.
In a known lithographic apparatus, the drive unit of the positioning mechanism for the substrate table comprises two linear Y-motors each of which comprises a stator which extends parallel to the Y-direction and is secured to a base of the positioning mechanism, and a translator (Y-slide) which can be moved along the stator. The base is secured to the frame of the lithographic device. The drive unit further comprises a linear X-motor which includes a stator which extends parallel to the X-direction, and a translator (X-slide) which can be moved along the stator. The stator is mounted on an X-beam which is secured, near its respective ends, to the translators of the linear Y-motors. The arrangement is therefore H-shaped, with the two Y-motors forming the “uprights” and the X-motor forming the “cross-piece”, and this arrangement is often referred to as an H-drive or gantry. U.S. Pat. No. 4,655,594 describes such an arrangement using hydraulic linear motors and mentions the possibility of using electric linear motors.
The driven object, in this case the substrate table, is provided with a so-called air foot. The air foot comprises a gas bearing by means of which the substrate table is supported so as to be movable over a guide surface of the base extending at right angles to the Z-direction.
To enable such an H-drive to actively control the yaw (rotation about the Z-axis) of the driven object, the two linear Y-motors are driven independently and the X-beam is usually mounted to the Y-translators by pivots (though U.S. Pat. No. 4,655,594 suggests that a rigid joint can be used). However, in this arrangement, very high loads are experienced at the pivots between the X-beam and the Y-slides. The pivots have to carry not only thrust reactions from the X-motor through the side bearings to the surrounding structure, but also the Y-motor actuation forces. This places very high demands on the elastic hinges commonly used for such pivots, especially when the yaw motion range is relatively large.
Further problems are encountered as the pivots on the X-beam cannot always be positioned on the line of force for the Y-motors, so that the side thrust bearing of the Y-slide has to accommodate both the X-reaction forces as well as the moment created by the Y-actuator forces and the offset between the pivots and the Y line of force. The resulting high loads in the known arrangement therefore leads to a design which can be cumbersome and heavy.